Just take a backup of the system using something like dar, create the VM as per normal, boot off a LiveCD in the VM, and restore to the virtual disk. Then just adjust /boot/loader.conf, /etc/rc.conf, and /etc/fstab to work with the new devices. Reboot the VM and you're done.

You may want to install a GENERIC kernel first, but it's not needed if you edit /boot/loader.conf before booting to enable the needed drivers.